It seems the toughest part of the second programming assignment is 
to take Bits [15:8] and get them into Bits [7:0} and then sign extend 
that value.  I have seen a number of "different" algorithms in the 
last couple of days that are really cumbersome.  No matter; as long 
as it works. I will leave it to 312 to teach you programming 
methodology.

I thought I would give you a couple of hints relative to this "toughest" 
part, one for each of two ways to solve the problem.

Hint for way #1:  If the value is in R3[15:8], and you execute the 
instruction ADD R4,R3,#0, are you then able to tell me the value of any 
of the eight bits in R3[15:8]?  Second, how can you exploit that 
information to get the value in R2[7:0], for example.

Hint for way #2: You want to move R3[15:8] into R2[7:0].  What should 
the value of bit R2[3:3] be, a 0 or a 1?  How can you figure that out?

Enough.  Feel free to use one of these hints or not, as you wish.

Good luck.

Yale Patt